Joachim Werther’s research while affiliated with Hamburg University of Technology and other places

This paper analyzes the capabilities of a pilot-scale chemical looping combustion plant firing wood biomass in two stages to efficiently achieve negative carbon dioxide emissions. The utilized in situ gasification-chemical looping combustion (iG-CLC) process isolates the oxygen supply via air from the fuel conversion itself with the help of two separate fluidized bed reactors and an oxygen carrier to supply the necessary oxygen for the combustion. As a result, a relatively pure stream of carbon dioxide and steam is generated. Thus, the process makes capturing carbon emissions more feasible since it eliminates the need for the cost- and energy-intensive separation of the produced gases. A major issue when using biomass in a chemical looping plant is the high amount of the volatiles exiting unconverted. This problem was mitigated by using a two-stage fuel reactor system. Two bubbling fluidized beds were arranged one upon the other. The lower stage, where the fuel is introduced, is used to release the volatiles and partly convert them. The remaining volatiles rise up into the second stage and are further converted to a high degree. A series of experiments were carried out with a 25-kWth pilot plant located at the Hamburg University of Technology. Gas concentrations were continuously measured after both stages of the fuel reactor to see the gradual conversion of the fuel gases. Additionally, carbon slip at the exhaust was measured to show the effectiveness. The experiments with the reactor concept showed promising results since already at a reactor temperature of 850 °C, the total oxygen demand needed to oxidize the combustible component in the exhaust gas was well below 2%. The carbon dioxide (CO2) capture efficiency when using German hardwood slightly decreased to 93–96% compared to 97% for German lignite. In the future, the reactor design must prove that it scales and that the efficiency can be further increased. Nevertheless, firing biomass with a two-stage iG-CLC process might allow a cost-efficient negative carbon dioxide emission while generating heat with relatively high efficiency. Therefore, it might be a sustainable alternative to generate heat in the future.

Due to the advantages of high energy density and convenient transportation, the use of pelletized biomass as fuel for power supply has gained interest over recent years. Chemical looping gasification (CLG) integrates the process of gasification and hot gas conditioning, with the objective to obtain syngas with a low tar amount. The work presents some experimental results using sawdust pellet with high-volatile and low-ash content as fuel, and a natural manganese-iron ore as oxygen carrier for CLG in a bubbling fluidized bed. Without the addition of external steam, the syngas, tar, and char collected at different temperatures (750–950 ℃) and oxygen carrier to biomass ratios (0.2–1.2) were investigated to determine the gasification performance of sawdust pellet. Three-phase product distributions during CLG process were identified under different operating parameters. The increase of reaction temperature enhances the gas production, with decreasing amount of liquid and solid. Also, it has significant influence on the main gases (CO, CO2, H2 and CH4) distributions in the syngas. Meanwhile, more Polycyclic-Aromatic Hydrocarbons (PAHs) are cracked to Mono-Aromatic Hydrocarbons (MAHs) and further decomposition into syngas, which is confirmed by further Gas Chromatography-Mass Spectrometer (GC–MS) analyses. The Mn/Fe-based oxygen carrier also exhibits a beneficial effect on catalytic cracking of PAHs into MAHs. As for the remaining char, a higher temperature is more sensitive to produce the char with more porous structure and brittle texture.

Due to the increasing transport cost of biomass and benefiting from the more efficient treatment of a compacted, dustless product, the use of pelletized biomass has gained interest over recent years in China. Chemical Looping Gasification (CLG) with circulating oxygen carriers provides a novel process, which integrates biomass gasification with the hot gas conditioning with the aim to obtain pure syngas with low tar amount. The study focuses on the CLG application using a single typical rice husk pellet as fuel which are characterized by high silicon dioxide in ash. Some experiments in a fluidized bed unit with the mixture of quartz sand and an active manganese ore as bed materials, were performed using a single rice husk pellet as fuel and steam as gasifying agent. The objectives of the work are to investigate its CLG performance and bottom ash characterization. Effects of gasification temperature (750-950 °C) and oxygen carrier-fuel ratio on syngas distributions, effective gas content and syngas yield, were investigated. The conversion of the rice husk pellet is much dependent on reaction temperature. A high temperature promoted tar cracking and gasification reactions, leading to a fast carbon conversion. The effective gas content (CO+H2+CH4) during gasification process were in the range of 74.2% to 79.9% under the temperature of 750 °C to 950 °C. Regarding the CLG application of rice husk pellet as fuel, much attention should focus on bottom ash, which was not separately during the process but still keeping the original pellet shape with some irregular pores inside the ash due to the formation of molten grains. The ash demonstrates a rigid skeleton-like structure. The trapped carbon particles inside the molten ash cannot be gasified, thus limiting the fuel conversion.

The Zhundong coalfield in Xinjiang, China, is the largest integrated coal basin newly found. The present work concentrates on the application of chemical looping combustion (CLC) with a Zhundong lignite, which is characterized by high sodium content. Some experiments in a laboratory scale fluidized bed facility with an active iron ore oxygen carrier, were performed using the lignite as fuel and CO2 as gasifying agent at a temperature of 900 °C, with the objective of investigating its combustion performance and sodium transfer behavior in CLC. Results indicate that the gasification reactivity of the three coals follows the order of German lignite > Zhundong lignite > American bituminous coal in the current experimental conditions. During reducing stage, the unique product of sodium transfer from coal to the fly ash is albite (NaAlSi3O8) due to the reactions between sodium and other coal ash. The sodium deposition on the oxygen carrier particles was not found. 40 reducing-oxidizing cycles were performed, and sodium accumulation in the bed materials with cycles was found due to some ash staying in the bed. However, the growth of bed particles due to the sodium accumulation was not observed by determining the particle size distributions of bed materials. This indicates that burning the high sodium Zhundong coal in the present conditions have no influence on the particle agglomeration. Finally, a literature survey was made and results indicate that the main sodium in the Xinjiang coal basin of China is water soluble with an average value of 64%. The pure salt of NaCl, as one common water soluble sodium phase in Zhundong coals, was introduced to a bed of iron ore particles at 900 °C with regard to investigate the influence of NaCl on fluidization stability. Based on the measurements of pressure drop, bed temperature and SEM-EDS, it was found that NaCl does not react with the iron ore but in fact only acts as glue between iron ore particles. Further, the sodium transfer routes in CLC of Zhundong coal with iron ore based oxygen carrier are given and some discussions are made with regard to practical operation. The corrosion problems on the heating surface in the air reactor can be significantly reduced compared to a conventional Zhundong coal fueled furnace, since most of sodium will release and be converted in the fuel reactor.

A novel flowsheet simulation environment is applied to simulate a system of interconnected fluidized bed reactors as they are used for the process of Chemical Looping Combustion of solid fuels. Dynamic models of the main process equipment are used in order to capture dynamic behavior of hydrodynamics inside an actual system, which delivers the experimental validation data. The process itself is carried out in a pair of strongly coupled fluidized bed reactors. Furthermore, a cyclone is used for gas-solid separation and two loop seal siphons prevent gas leakages from one reactor to another. The experimental plant operated at Hamburg University of Technology, which is modeled here, comprises a circulating fluidized bed air reactor and a two-stage bubbling bed fuel reactor. Operation of the plant is carried out at ambient condition and so are the simulations.

To investigate the influence of the fuel characteristics on the conversion behavior in a chemical-looping combustion facility, lignite coal dust (d90,3=233 μm) and two fractions of bituminous coal with different particle sizes (fine fraction d90,3=163 μm, coarse fraction d50,3=707 μm) were used as solid fuel. To improve the conversion performance, a pilot plant with a rated power of 25 kW was constructed with a two-stage fuel reactor. The influence of the fuel composition, particle size, and the presence/absence of elemental oxygen on the conversion in the fuel reactor are presented. The used oxygen carrier was produced by the impregnation of γ-alumina oxide with copper oxide, which is able to release gaseous oxygen, but loses this ability because of deactivation. The lignite dust shows a very good conversion performance and carbon capture efficiencies over 95 % as well as oxygen demands below 2 %. Both bituminous coal fractions have a good performance with regard to fuel conversion and oxygen demand but they suffer from a high carbon slip. Hence the carbon capture efficiency is around 60 % for the fine fraction and 40 % for the coarse one. The performance improvement as a result of the second stage was investigated separately, and we proved that it enhances the overall conversion. In addition, the oxygen carrier generated a favorable reaction environment by releasing elemental oxygen in the second stage of the fuel reactor.

Silica in the form of clay present in the bauxite ore causes lot of complications in the Bayer hydrometallurgical process for the production of alumina. Therefore, bauxite is processed and washed in trommels to disintegrate and remove the clay before sending the ore as a feed to the Bayer process. Estimation of ore washing potential and the selection of cut size is essential for the establishment of a bauxite washing plant. In the current research study, a simple ultrasonic washing technique has been developed for the removal of clay and to determine the potential of an industrial washing process at various cut sizes. Four feed size fractions of clayey bauxite ore were washed in laboratory for 1, 3, 5 and 6 min using ultrasounds. After 5 min, the steady state washing conditions were observed. It was found that at 36 micron cut size, 70% mass and 77.5% alumina recovers whereas 69% of total silica moved towards the tailings streams. Cut size sensitivity shows that by increasing the cut size from 36 micron to 100 micron, nearly 5% reduction in the recoveries of total mass and alumina was observed whereas the recovery of silica slightly reduced in the concentrate.

This review of industrial applications of fluidized-bed reactors is focused on the fluid catalytic cracking (FCC) process and the fluidized-bed combustion. Both processes have a tremendous economic importance. In the FCC process, the main challenges are the increase of runtime, the adaption of the product spectrum to regional requirements of the market, and the adaption to heavier feedstocks. In fluidized-bed combustion the main trends are towards larger units sizes, higher efficiency, and further reduced emissions while keeping the boiler as flexible as possible with regard to the mixture of fuels.

Clay impurities associated with bauxite negatively affect the Bayer process for alumina production. These impurities should be removed as far as possible by a beneficiation technique before the ore is used as feed for the Bayer process. In this current investigation, bauxite washing was conducted in the laboratory. Bauxite washing is a physical process that causes the disintegration and deagglomeration of the clay matrix, and bauxite is liberated from the clay (mainly rich in silica). Subsequently, separation occurs with the assistance of wet screening at a predetermined cut size. Three techniques were investigated in the laboratory: drum washing, water-jet washing, and ultrasonic washing. Various operating parameters were investigated for drum washing and water-jet washing, including materials retention time, drum rotation speed, solid concentration, water-jet spray duration, pressure, and height. We concluded that the retention time of bauxite inside the drum at a solid concentration of 55wt% and a drum rotation speed of 31 r/min is the dominant parameter for the removal of clay from the bauxite surface. © 2014, University of Science and Technology Beijing and Springer-Verlag Berlin Heidelberg.